Certain equipment, such as for example video equipment (for example cameras or video recorders), is able, once synchronized with respect to a reference time base, of providing synchronized data (defining for example video images). This synchronization is done by transmitting to the equipment a synchronization signal, which is for example called “Genlock” in the (non-limiting) case of video equipment.
When the transmission of the synchronization signal is done by means of a dedicated cable, for example of coaxial type, no other signal flows in this cable. Consequently, the lag in transmitting the synchronization signal up to the various items of equipment to be synchronized is constant and devoid of jitter. On the basis of the synchronization signal received, each receiver item of equipment is able to reconstruct a timing clock (or reference time base or else reference clock signal) specific to its operation and guaranteeing that each data set (such as an image) that it generates is strictly in phase with each data set generated by each other item of equipment which is the subject of the same synchronization. Thus, two cameras can for example generate video contents which differ but are strictly in phase and in frequency with respect to one another. It is recalled that the phase and frequency of a clock constitute what is called its timing.
When the equipment is connected to a communication network introducing transmission lags and jitters that vary from one item of equipment to another, as is the case in particular for a so-called packet switching network, such as a wire-based (Ethernet) or non-wire-based IP network, it is no longer possible to transmit the synchronization signal. A sampled ramp signal which is representative of the synchronization signal is then transmitted on the network part.
More precisely, on the send side, information making it possible to recover a reference clock signal and content ticks (for example image ticks) is extracted from the synchronization signal (for example Genlock), which is delivered by a master item of equipment. The reference clock signal supplies first and second counters delivering first and second synchronous ramp signals representative of the number of ticks of the reference clock signal that have been delivered respectively since the last recovered content tick and since a last reference tick. The value of the first counter is set to zero each time that a content tick is recovered. A third counter meters the number of zero settings of the first ramp signal and generates a reference tick each time that this number is equal to a chosen threshold. The value of the second counter (generally termed “PCR” (for “Program Clock Reference”)) is set to zero each time that a reference tick is generated. The second ramp signal is sampled according to a sampling frequency (which is generally provided by the network) and the resulting samples are transmitted via the network to the receiver items of equipment by means of frames of packet(s).
It will be noted that in the case of video contents, the period of the content ticks is for example equal to 40 ms in the case of a 625 lines standard.
On the receive side, the samples received via the network are used to synchronize a phase-locked loop (or PLL) and therefore to reconstruct the starting reference clock signal. More precisely, the phase-locked loop reconstructs the reference clock signal on the basis of the second sampled ramp signal transmitted, then it reconstructs a second ramp signal identical to, and in phase with, that having been sampled send side. Processing means are charged with initializing the value of a synchronization counter, synchronous with respect to the second reconstructed ramp signal, on the basis of the latter and of the reconstructed reference clock signal.
The second ramp signals (PCR), sampled send side at regular intervals (sampling period) arrive at irregular intervals on receive side, predominantly because of the jitter which is introduced by transporting them (for example over IP). These PCR signals are again taken into account at regular intervals (Tsamp) on the receive side. The phase-locked loop (PLL) is charged with filtering the jitter related to the sampling instant of the PCR counting ramp by the sampling signal of period Tsamp, so that a second ramp signal which evolves strictly in a manner synchronous with the second ramp signal (PCR) generated send side is retrieved as output from the counter (receive side). The inaccuracy between the sampling instants on send side and on receive side is absorbed by the PLL whose bandwidth is appropriate. Therefore the timing of the reference clock signal reconstructed receive side is identical to that generated on send side, both in frequency and in phase.
Once the timing has been reconstructed, it is necessary to reconstruct the first ramp signal and synchronize it with respect to the second reconstructed ramp signal. Accordingly, the same zero-setting period is used as that used on the send side (for example 40 ms). Then, each time the first ramp signal is set to zero a content tick is generated, and the initial synchronization signal is reconstructed on the basis of the content ticks and the reference clock signals reconstructed.
This synchronization mechanism is disclosed in international application PCT FR2007/050918.
By doing this way, equipments driven by such a reference station produce synchronously data streams. But the system described above doesn't cope with the data delivery duration after the stream production which can be a problem. This problem appears for example when a sound and a video signals are both simultaneously acquired by a microphone and a camera synchronized as described above. Both, video and audio streams are acquired and delivered synchronously. When such streams are sent to a TV set, if delivery duration of the video stream between the camera and the TV set isn't exactly the same than the delivery duration between the microphone and the TV set, an audio lip sync effect appears on the TV set caused by the delay between the two streams. The reconstruction of synchronization signals on camera and on microphone is not a guarantee that both streams are perfectly in phase at TV set level.
Another illustration of the problem is encountered when two video streams delivered by two synchronized cameras are sent to a video mixer. Due to a difference of delivery duration, for example caused by a difference of transport and/or processing duration on two different data paths, one cannot make a clean transition between two video streams.
One of the goals of the present invention is to remedy the concerns connected with the delays due to various data delivery duration, when data streams are produced between various synthesized synchronization signals in the prior art, by providing a mechanism that compensate these delays.